Drying of the hair implies that the water held by the hairs is made to evaporate by means of a flow of hot air. This is obtained by means of a fan which supplies the air flow, the latter being heated by making passing contact with heating means. At the start of the evaporation process, the water held by the hairs forms a thermal screen which prevents burning of the scalp. In proportion as the water evaporates, it acts less and less as a screen, and the hot air flow may cause a burning sensation, and may even cause burning of the scalp as well as drying-out and deterioration of the hair.
To avoid the hair and the scalp being subjected to excessive temperatures during the drying process, the idea has arisen to measure the hair temperature.
One of the methods using detectors without direct contact with the hair is described in the document GB 2.093.343 A. This method measures at a distance the temperature of the hair by means of an infrared sensor which is positioned outside the body of the hair drier. An optical system renders it possible to focus on the location of the hair which is receiving the drying treatment. The infrared detector acts on the heating means when a given temperature is reached.
A hair drier of this type, however, has certain disadvantages. In particular, the focusing point of the infrared sensor must coincide with the heated zone. It is necessary therefore to keep the hair drier at a fixed distance in order to obtain a correct operation. The measurement is incorrect owing to the parallax with too short or too long distances then, and the action of the detector on the heating means is not carried out correctly, which leads to the burns described above. This is especially disadvantageous when a third person handles the hair drier: a hairdresser on a client, hair drying on a child, or when a person dries long hairs, in which case there is no sensorial alarm.
Another type of appliance to which the invention relates is formed by radiators which generate a flow of hot air. Such a radiator may be so arranged that a body can be subjected to the action of the flow of hot air whereby the temperature assumed by said body becomes abnormally high, which may lead to its deterioration.
These situations arise, for example, with:
a cloth held in front of a radiator, PA1 a mobile radiator being placed close to an object: curtain, piece of furniture, or other objects,
or any other similar situation.